To obtain hydrocarbons such as oil and gas, boreholes are drilled by rotating a drill bit attached to a drill string. The drill bit is typically mounted on the lower end of the drill string as part of a bottom-hole assembly (BHA) and is rotated by rotating the drill string at the surface and/or by actuation of down-hole motors or turbines. A variety of sensors employed in the drill string are used to monitor various down-hole conditions, such as pressure, spatial orientation, temperature, gamma ray count, etc., that are encountered while drilling. The use of sensors during the drilling operation to provide information related to positioning or steering the drill, such as direction, orientation, gamma, and drill bit information, is referred to as “Measurement While Drilling” (MWD). The phrase “Logging While Drilling” (LWD) is often used to using sensors for petrophysical or geological measurements during drilling. As used herein, “MWD” will also be used to encompass LWD applications unless otherwise specified. Regardless of whether MWD or LWD sensors are used, in order for the drilling to benefit from “real-time” information, sensor data (either raw or processed) must be transmitted to the surface, typically from deep underground.
Although a variety of transmission methods are known, one of the most common is referred to as electromagnetic (EM) telemetry. A down-hole EM transmitter is used to create very low frequency EM carrier waves, which are modulated in order to carry information (such as sensor data). These low-frequency waves will travel through the earth surrounding the borehole to the surface where the signal can be detected, typically by measuring the induced electric potential difference between the drill rig and a grounding rod located in the earth some distance away. These signals are received by a receiver at the surface and deciphered by a control circuit or processor.
Specifically, the EM carrier waves are generated by applying an alternating electric current across an electrically isolated (nonconductive) portion of the drill string referred to as the “gap sub.” This allows the upper and lower portions of the drill string (electrically isolated from each other by the gap sub) to function as a dipole antenna so that an alternating current applied to the two isolated portions of the drill string results in the generation of an EM carrier wave, which can be modulated to transmit digital information.
Generally, the gap sub assembly must electrically insulate the upper and lower sections of the drill string and yet be structurally capable of carrying high torsional, tensile, compressive, and bending loads. This is especially true for directional drilling where the drill string, and in particular the sub gap connector, is typically subjected to extreme torsional, compression, tension, and bending forces. Such extreme forces can result in gap sub connection failure, usually at the weakest point in the subassembly.
Although a number of different gap subs are known in the prior art, there still exists a need for a gap sub assembly that can withstand the rigors of directional drilling applications, while still being cost effective to manufacture and easy to deploy at the drill site.
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